Stop doing lb_type dispatch in delete_server

This should be done after #772, #773. Closes #767.
So this code changes the delete_server behavior when the lb type is RackConnectV3 -- previously it would be an error, but now it should actually succeed since remove_from_load_balancer handles RackConnectV3. I haven't looked *too* closely, but maybe there should be a unit test for passing a RCv3 load balancer to delete_server?